The insulation resistance value of the transformer is evaluated based on the resistance value at 1 minute. The insulation resistance of the coil at 20℃ should not be less than 2000 MΩ. The insulation resistance of the core should not be less than 200 MΩ. If it is lower than the above requirements, the cause should be investigated. When the insulation resistance is low, it is generally related to the drying quality of the transformer body and the quality of the transformer oil. When the insulation resistance of the transformer is low, filtering and hot oil circulation treatment can be carried out, and the effect is usually quite obvious.
If the oil filtration effect is not obvious, the transformer body needs to be dried again. When the air humidity is very high in summer and the assembly time of the transformer is too long, the transformer body is exposed to the humid air for a long time, and the insulation resistance will decrease significantly. The faster the assembly is completed, the shorter the exposure time will be, which is better.
Common issues in insulation resistance measurement:
The absorption ratio measurement of transformers often fails to meet the standard of 1.3. This requires a comprehensive analysis. When the insulation resistance value is extremely high, the absorption ratio often fails to meet the requirements. However, this does not indicate that the insulation is defective or damp; rather, it is a sign of good insulation condition. The temperature of the transformer can be increased to make a judgment. When the temperature rises, the insulation resistance value decreases while the absorption ratio increases.
When the absorption ratio fails to meet the requirements, the polarization index measurement can be conducted. When the polarization index reaches 1.5, it indicates good insulation. When the insulation resistance value is very low and the absorption ratio and polarization index fail to meet the requirements, it indicates that the transformer is severely damp and treatment should be carried out.
The insulation resistance of transformers in summer is often not very high. This is also related to the moisture on the surface of the porcelain bottles (terminals). When measuring insulation, shielding is applied on the surface of the porcelain bottles. The shielding ring is connected to the shielding terminal of the multimeter, which can eliminate the influence of surface moisture.
In addition, the insulation of the measurement leads of the insulation resistance tester should also be good. Before measurement, conduct self-checks using the instrument to confirm the insulation condition of the test leads and eliminate the influence of the test leads on the insulation resistance of the insulation resistance tester. Transformers often have problems such as poor insulation of the core and accessories, or even insulation reaching zero. The causes of such problems are mostly due to moisture in the insulation parts of the core, presence of foreign objects in the transformer, and displacement of the fixed insulation parts.
Post time: Mar-27-2026